Dar

This article is a draft

This is not a complete article: This is a draft, a work in progress that is intended to be published into an article, which may or may not be ready for inclusion in the main wiki. It should not necessarily be considered factual or authoritative.

Parent page: Storage and file management

The dar (stands for Disk ARchiver) utility was written from the ground up as a modern replacement to the classical Unix tar tool. First released in 2002, dar is open source, is actively maintained, and can be compiled on any Unix-like system.

Similar to tar, dar supports full / differential / incremental backups. Unlike tar, each dar arhive includes a file index for fast file access and restore -- this is especially useful for large archives! dar has built-in compression on a file-by-file basis, making it more resilient against data corruption, and you can optionally tell it not to compress already highly compressed files such as mp4 and gz. dar supports strong encryption, can split archives at 1-byte resolution, supports extended file attributes, sparse files, hard and symbolic (soft) links, can detect data corruption in both headers and saved data and recover with minimal data loss, and has many other desirable features. On the dar page you can find a detailed feature-by-feature tar-to-dar comparison.

Where to find dar

Since dar can be compiled on the command-line, you can install it easily on Linux and MacOS. On Compute Canada clusters a slightly out-of-date version can be found in /cvmfs:

[user_name@localhost]$ which dar
/cvmfs/soft.computecanada.ca/nix/var/nix/profiles/16.09/bin/dar
[user_name@localhost]$ dar --version
dar version 2.5.3, Copyright (C) 2002-2052 Denis Corbin
...

If you want a newer version, you can compile it from source (replace 2.6.3 with the latest version number):

[user_name@localhost]$ wget https://sourceforge.net/projects/dar/files/dar/2.6.3/dar-2.6.3.tar.gz
[user_name@localhost]$ tar xvfz dar-*.gz && /bin/rm -f dar-*.gz
[user_name@localhost]$ cd dar-*
[user_name@localhost]$ ./configure --prefix=$HOME/dar --disable-shared
[user_name@localhost]$ make
[user_name@localhost]$ make install-strip
[user_name@localhost]$ $HOME/dar/bin/dar --version

Using dar manually

Basic archiving and extracting

Let's say, in the current directory you have a subdirectory test. To pack it into an archive, you can type in the current directory:

[user_name@localhost]$ dar -w -c all -g test

This will create an archive file all.1.dar, where all is the base name and 1 is the slice number. You can break a single archive into multiple slices (below). You can include multiple directories and files into an archive, e.g.

[user_name@localhost]$ dar -w -c all -g testDir1 -g testDir2 -g file1 -f file 2

Please note that all paths should be relative to the current directory.

To list the archive's contents, use only the base name:

[user_name@localhost]$ dar -l all

To extract a single file into a subdirectory restore, use the base name and the file path:

[user_name@localhost]$ dar -R restore/ -O -w -x all -v -g test/filename

The flag -O will tell dar to ignore file ownership. Wrong ownership would be a problem if you are restoring someone else's files and you are not root. However, even if you are restoring your own files, dar will throw a message that you are doing this as non-root and will ask you to confirm. To disable this warning, use -O. The flag -w will disable a warning if restore/test already exists.

To extract an entire directory, type:

[user_name@localhost]$ dar -R restore/ -O -w -x all -v -g test

Similar to creating an archive, you can pass multiple directories and files by using multiple -g flags. Note that dar does not accept Unix wild masks after -g.

Incremental backups

You can create differential and incremental backups with dar, by passing the base name of the reference archive with -A. For example, let's say on Monday you create a full backup named monday:

[user_name@localhost]$ dar -w -c monday -g test

On Tuesday you modify some of the files and then include only these files into a new, incremental backup named tuesday, using monday archive as a reference:

[user_name@localhost]$ dar -w -A monday -c tuesday -g test

On Wednesday you modify more files, and at the end of the day you create a new backup named wednesday, now using tuesday archive as a reference:

[user_name@localhost]$ dar -w -A tuesday -c wednesday -g test

Now you have three files:

[user_name@localhost]$ ls *.dar
monday.1.dar     tuesday.1.dar    wednesday.1.dar

The file wednesday.1.dar contains only the files that you modified on Wednesday, but not the files from Monday or Tuesday. Therefore, the command

[user_name@localhost]$ dar -R restore -O -x wednesday

will only restore files that were modified on Wednesday. To restore everything, you have to go through all backups in the chronological order:

[user_name@localhost]$ dar -R restore -O -w -x monday      # restore the full backup
[user_name@localhost]$ dar -R restore -O -w -x tuesday     # restore the first incremental backup
[user_name@localhost]$ dar -R restore -O -w -x wednesday   # restore the second incremental backup

Limiting the size of each slice

To limit the maximum size of each slice in bytes, use the flag -s followed by a number and one of k/M/G/T. For example, for a 1340 MB archive, the command

[user_name@localhost]$ dar -s 100M -w -c monday -g test

will create 14 slices named monday.{1..14}.dar. To extract from all of these, use their base name:

[user_name@localhost]$ dar -O -x monday

Using dar via functions

Using dar would be much easier if you did not have to memorize and specify all the flags and the right syntax on the command line. Here we provide several bash functions for easy backup. Please note that these functions assume that you are below your quota (so you can write files!), have read and write permissions, i.e. all the common-sense assumptions. It is your job to ensure that this is the case, and that dar archived/restored your files correctly before you delete the originals. In other words, please test everything before including these functions into your workflow.

Limiting the number of files in each slice with multidar

Using copy and paste, define the following function in your shell, or put this definition into your $HOME/.bashrc file and then source it with source ~/.bashrc:

function multidar() {
    if ! [ $# = 2 ]; then
	echo Usage: multidar sourceDirectory maxNumberOfFilesPerArchive
    else
	sourceDirectory=$1
	maxNumberOfFilesPerArchive=$2
	if which dar 2>/dev/null; then
	    echo great, I found dar at $(which dar)
	    find $sourceDirectory -type f > .fullList
	    sed -i -e '/DS_Store/d' .fullList
	    sed -i -e 's/\/\//\//' .fullList
	    split -a 3 -l $maxNumberOfFilesPerArchive .fullList .partial
	    for i in .partial*; do
		echo archiving from $i to ${sourceDirectory%?}-${i:8:3}
		dar -w -c ${sourceDirectory%?}-${i:8:3} --include-from-file $i
 		/bin/rm -rf $i
 	    done
 	    /bin/rm -rf .fullList*
	    ls -lh ${sourceDirectory%?}*.dar
	else
	    echo please install dar
	fi
    fi
}

Now, running the command without arguments will show you the syntax:

[user_name@localhost]$ multidar
Usage: multidar sourceDirectory maxNumberOfFilesPerArchive

Let's assume that we have 1000 files inside test. Running the command

[user_name@localhost]$ multidar test 300

will produce four archives, each with its own basename and no more than 300 files inside. To restore from these archives, use a bash loop:

[user_name@localhost]$ for f in test-aa{a..d}
                       do
                         dar -R restore/ -O -w -x $f
                       done